1. Field of the Invention
The present invention relates to a dyeing device and to processes using indigo and/or other colorants, to which the warp yarn chain and/or fabrics are exposed in a continuous manner. In particular, the present invention relates to a continuous dyeing device and to processes in dyeing machines and/or plants that operate with reduction baths and at high or low temperatures. A typical application of this dyeing technology is that of the continuous dyeing of the warp chain for denim fabric, using indigo or other colorants, such as sulphur, indanthrene and reactive based colorants.
2. Description of Related Art
Denim is the fabric normally used for the manufacturing of jeans and other sportswear articles, and is the fabric used in the greatest quantity throughout the world. Traditional denim is manufactured by weaving pre-dyed cotton yarn; in particular, only the warp is dyed in a continuous manner with indigo or other colorants, whereas the fill is used in its raw state.
Typically, the dyeing of the warp chain for denim fabrics is performed using both rope and shift systems in open vats at low temperature, using indigo, an ancient natural colorant that is currently produced by synthesis. The dyeing method required for the application of indigo colorant on cotton yarn is tailored to this colorant, because said colorant has a relatively small molecule with reduced affinity for cellulose fiber, and, therefore, for the application on said fiber, it needs to be not only chemically reduced in an alkaline bath (in leuco form) but also a plurality of impregnations are required with squeezing and oxidation stages in the air, between each bath.
Therefore, in order to obtain medium or dark blue denim, the yarn must be subjected to a first dyeing process (impregnation, squeezing, oxidation) immediately followed by other over-dyeing stages: the greater the number of stages, the darker the color, and the stronger the solidity of the required color.
The above described dyeing process is currently applied to the warp chain, using indigo in all known machines and installations in a continuous manner, for both rope and shift systems. More precisely, the machines described above are normally composed of 2 or more pre-treatment vats, 8 or more dyeing vats that include the related squeezing and oxidation units, followed by 3 or more units for final washing stages. The dyeing vats are connected to each other by a circulation system for mixing, changing and strengthening the dye bath, according to a known system, not described herein.
It is known that the dyeing in these machines is performed in an alkaline environment and with a calculated excess of sodium hydrosulphite and that, as a result of the bath/air contact, the sodium hydrosulphite reacts with the oxygen in the air losing its reducing capacity towards the indigo.
Because sodium hydrosulphite oxidizes easily, metered quantities of hydrosulphite must be added to the dyeing bath to contrast this loss.
This reintegration of the sodium hydrosulphite with stechiometrically corresponding amounts of caustic soda must be performed with regularity and precision, in order to keep the dyeing capacity of the bath unaltered, and to guarantee constant and repeatable results.
Among the aforesaid machines, those used in shift dyeing systems are then connected in line within dyeing installations to a slashing machine, that performs the slashing, the drying, and the winding of the dyed yarn on a beam, ready to be placed on the loom.
The aforesaid dyeing machines must be constructed respecting determined basic parameters in relation to the immersion and oxidation times, and to permit the yarn to absorb the dye in the best possible conditions, and after squeezing, to be completely oxidized before entering into the following bath, so that the color can be intensified, in other words so that the color tone can be darkened.
The average immersion time for the yarn in the dyeing bath is normally approximately 8-12 seconds, while the oxidation time after squeezing is approximately 60 seconds, which means that the yarn must remain exposed to air for 60 seconds before it can be immersed again in the following vat, and then this process is repeated for all the vats.
A reduced oxidation time is possible only with machines equipped with oxidation intensification devices, such as those described in patent no. EP533286 by Applicant.
The average dyeing speed can be calculated as approximately 30 meters per minute, meaning an average length of 6-8 meters of yarn immersed in each bath, while the length of yarn exposed to air between one bath and another is at least 30 meters and more. Therefore, considering a machine with eight dyeing vats as a basic installation, a substantial length of yarn runs through the dyeing baths and the relative oxidation devices, since, by multiplying: 6 m×8+30 m×8, this equals 288 meters.
These 288 meters of yarn, added to the much shorter lengths of yarn being processed in the other component machines on the same line (dyeing machine+slasher) reach a total of 400-500 meters, making the line difficult to control, and at each batch change, certain quantities are considered as lost, because the dyeing is not uniform and because of problems connected to the start-up of a new batch.
The aforesaid machines must also be adaptable to dyeing processes with other colorants, such as the aforesaid sulphur, indanthrene and reactive based colorants, which require different methods from those used for indigo. These machines, which use different processes from those used for indigo dyeing, require flexibility and adaptability, so that cost increases connected to the installation of specific dyeing systems can be contained.